The overall goal of the proposed program of research is to characterize the process by which listeners recognize words in fluent speech. Word recognition in this context, which comprises nearly all of our exposure to words in sentences, requires a remarkably sophisticated system that can rapidly and accurately identify words in real-time from among the thousands of alternatives in our mental lexicon. This process entails mechanisms by which acoustic/phonetic information is combined to create auditory word-forms that, when linked to meanings, serve as the entries in the lexicon. Unresolved issues for theories of spoken word recognition are (a) the units used to represent auditory word-forms in the lexicon, (b) when during the temporal unfolding of speech these units are used to access lexical candidates, (c) how these units are organized to represent words in the lexicon, and (d) how the mapping of sounds to meanings is reorganized when confronted with systematic shifts in the sound patterns of the input. We address these questions in studies of adults and children using standard judgment tasks (e.g., lexical decision), and eye movements, which are remarkably sensitive to the time course of speech and lexical processing, thus allowing for a detailed mapping of how asynchronous acoustic cues are integrated and how lexical candidates are evaluated dynamically. Many of the proposed experiments use miniature artificial languages to enable precise control over the sound patterns and frequencies of occurrence of words that cannot be achieved in natural languages. Studies using artificial lexica also enable us to study the process of lexical learning to determine how new words enter into, and may reorganize, the current lexical representation. The proposed experiments focus on (a) how context modulates the effects of lexical competitors, and (b) the dynamics of cue integration and lexical reorganization. The expected outcomes of this program of research will reveal fundamental aspects of spoken word recognition in adults and children that have important implications for models of lexical organization and access, for the mechanism by which the efficiency of lexical access is optimized in normal listeners, and for the effects of experience on the reorganization of both mature and immature lexicons. Successfully addressing these specific aims will provide a base of knowledge from which diagnostic tests and treatment regimens for children and adults with delays or deficits in spoken word recognition can be developed. The research outlined in this grant proposal bears on two of the four research areas outlined in the NICDC draft Strategic Plan for 2006-08: II. Development, Deterioration, Regeneration, and Plasticity of processes mediating communication, and III. Perception, Cognition, and Sensorimotor processing in normal and disordered communication.